This invention involves a process for making amines, particularly dimethylamine, in which methanol and/or dimethylether and ammonia are contacted in the presence of a selected zeolite catalyst.
Methylamines are generally prepared in industrial quantities by continuous reaction of methanol and ammonia in the presence of a silica-alumina catalyst. The reactants are typically combined in the vapor phase, at temperatures in the range of 300.degree. to 500.degree. C., and at elevated pressures. Trimethylamine (TMA) is the principal component of the resulting product stream, accompanied by lesser amounts of monomethylamine (MMA) and dimethylamine (DMA). From a commercial standpoint, the most valued product of the reaction is dimethylamine, in view of its widespread industrial use as a chemical intermediate. Accordingly, a major objective of those seeking to enhance the commercial efficiency of this process has been to improve overall yields of dimethylamine, and to a lesser extent, monomethylamine, relative to trimethylamine. Among the approaches taken to meet this objective are recycling of trimethylamine, adjustment of the ratio of methanol to ammonia reactants, and use of selected dehydrating or aminating catalyst species. Due to the commercial importance of the process, an extensive compendium of patents and other contributions to the technical literature has resulted. Representative references generally relevant to the field of the present invention are summarized in the following paragraphs.
Swallen, U.S. Pat. No. 1,926,691, discloses a process for producing dimethylamine by disproportionating monomethylamine over dehydrating or aminating catalysts such as alumina, silica, thoria, aluminum silicate or partially dehydrated aluminum trihydrate.
Arnold, U.S. Pat. No. 1,992,935, describes a process for catalytic synthesis of amines from alcohols and ammonia which employs as catalyst a dehydrating oxide, e.g., alumina, deposited on the surface of a porous, rigid gel, e.g., silica gel. Arnold, U.S. Pat. No. Re. 19,632, discloses a process improvement in which trimethylamine is introduced with the methanol and ammonia reactants to shift reaction equilibrium in favor of dimethylamine production.
Johnson, British Pat. No. 422,563, discloses a process for producing aliphatic amines involving heating an alcohol or ether under a pressure of more than about 50 atmospheres in the presence of a "catalyst capable of splitting off water" (e.g., alumina), with an excess of ammonia and optionally with addition of primary amine to the reaction mixture.
Goshorn, U.S. Pat. No. 2,349,222, discloses use of granular alumina coated with one or more oxides of nickel, cobalt, or chromium as a catalyst for alkylation of ammonia to produce alkyl amines. Goshorn, U.S. Pat. Nos. 2,394,515 and 2,394,516, discloses use as catalyst of an aluminum salt or oxide coated with silica and vanadium or molybdenum oxide.
Smith, U.S. Pat. No. 2,456,599, discloses a process improvement wherein water is added to a reactant feed mixture of methanol and ammonia to repress formation of tertiary amine in favor of primary and secondary amine.
Markiewitz, U.S. Pat. No. 3,278,598, discloses use of a rhodium, palladium, or ruthenium cocatalyst in conjunction with Raney metals to increase production of secondary amines from the reaction of alcohols and ammonia.
Rostelli et al., A. I. Ch. E. Journal 12:292 (1966) describe studies of transmethylation reactions of monomethylamine and dimethylamine over montmorillonite, a hydrated magnesium or calcium oxide-containing aluminosilicate having a porous lattice structure. For transmethylation of monomethylamine, this work indicated that reaction rate was directly proportional to reactant partial pressure, indicating that the rate-determining event is adsorption of reactant to the catalyst surface.
Hamilton, U.S. Pat. No. 3,384,667, describes alkylation of ammonia in the presence of a dehydrated crystalline aluminosilicate catalyst having pores of a diameter permitting absorption of primary and secondary, but not tertiary, amine products.
Leonard, U.S. Pat. No. 3,387,032, discloses a process for reacting ammonia with methanol and/or dimethylether in the presence of a catalyst consisting of a silica gel base impregnated with 10-15% alumina which is first steam-deactivated and then treated with silver, rhenium, molybdenum, or cobalt ions to promote selectivity for dimethylamine.
Kaeding, U.S. Pat. No. 4,082,805, discloses use of a crystalline aluminosilicate or zeolite catalyst having the structure of ZSM-5, ZSM-11 or ZSM-21 in a process for producing amines by reaction of ammonia with C.sub.1 -C.sub.5 alcohols at elevated temperatures and pressures.
Parker et al., U.S. Pat. No. 4,191,709, describe the use of a hydrogen form of zeolite FU-1 or zeolite FU-1 in which some or all of the protons have been replaced by bivalent or trivalent cations.
Weigert, U.S. Pat. No. 4,254,061, discloses a process in which production of monomethylamine is enhanced by reacting methanol and ammonia in amounts sufficient to provide a C/N ratio of 0.5 to 1.5 over a catalyst selected from
(a) mordenite wherein the primary cation is Li, Na, HNa having at least 2% Na by weight, K, Ca, Sr, Ba, Ce, Zn or Cr; PA0 (b) ferrierite wherein the primary metal cation is Li, Na, K, Ca, Sr, Ba, Ce or Fe; PA0 (c) erionite ore; PA0 (d) calcium erionite; and PA0 (e) clinoptilolite ore, PA0 (a) mordenite wherein the primary cation is Na, HNa having at least 2% Na, Mg, Ca, Sr or Ba; PA0 (b) ferrierite wherein the primary metal cation is Na, K, Mg, Ca, Sr or Ba; PA0 (c) clinoptilolite; and PA0 (d) phillipsite, PA0 Meier et al., Atlas of Zeolite Structure Types (International Zeolite Assn. 1978); PA0 Mumpton, "Natural Zeolites" in Reviews in Mineralogy 14:1 (1977); PA0 Smith, "Origin and Structure of Zeolites" in Zeolite Chemistry and Catalysis, ACS Monograph 171 (American Chemical Society, 1976).
at a temperature of 250.degree.-475.degree. C. and a pressure of 7-7000 kPa, a contact time, normalized to 7 kPa of 0.1 to 60 seconds and a methanol conversion of 15-95%.
Ashina et al., Japanese published Patent Application No. 56-53887, and Mochida et al., Journal of Catalysis 82:313 (1981), also disclose use of mordenite zeolites to enhance production of dimethylamine in closely related variants of the process disclosed by Weigert.
Weigert, U.S. Pat. No. 4,313,003, discloses an improved process for disproportionating monomethylamine to dimethylamine and ammonia, comprising passing monomethylamine over a crystalline aluminosilicate catalyst selected from
at a temperature of 250.degree.-475.degree. C. and a pressure of 7-7000 kPa, at a feed rate of 0.1-10 g of monomethylamine/g of catalyst per hour, and at a monomethylamine conversion of 15-75%.
Cochran et al., U.S. Pat. No. 4,398,041, describe a process for converting C.sub.1 -C.sub.4 alcohols to a non-equilibrium controlled distribution of primary, secondary, and tertiary alkylamines. The process disclosed involves passing a mixture of reactant alcohols and ammonia into a first conversion zone containing a "shape-selective" crystalline aluminosilicate catalyst having a pore size selective for mono- and disubstituted alkylamine products; dividing the resulting product stream; passing one portion of this product stream to a second conversion zone containing another catalyst having a different pore size distribution; and combining the remaining portion of the first product stream with the product stream of the second conversion zone to yield a non-equilibrium controlled product distribution. The zeolite catalysts disclosed by this reference include 5A zeolite, REY zeolite, H-chabazite-erionite, H-erionite, H-mordenite, and H-Y zeolite. Deeba et al., published European Patent Application No. 0085408, disclose a method for improving methanol conversion rates comprising reacting methanol and ammonia over a highly acidic dehydrated aluminosilicate catalyst having a silica to aluminum ratio of at least 2.0 and manifesting microporous diffusivity for methylamines.
Deeba et al., U.S. Pat. No. 4,434,300 disclose a method for improving methanol conversion rates in the reaction of methanol and ammonia to produce methylamines which comprises conducting the reaction in the presence of a macroporous, highly acidic aluminosilicate.
Tompsett, U.S. Pat. No. 4,436,938, discloses a process for making methylamines comprising reacting methanol and/or dimethylether over a binderless zeolite A catalyst, preferably a binderless zeolite 5A catalyst.
Currently, methylamines are produced using an adiabatic plug flow reactor. Although specific conditions do vary depending upon ammonia feed ratio and amount of product recycle, reactor inlet temperatures are generally run from about 310.degree. C. to about 340.degree. C., and outlet temperatures are preferably about 400.degree. C. to about 430.degree. C. The difference between inlet and outlet temperatures is due to exothermicity of the reaction and is moderated by recycling of ammonia and trimethylamine. The foregoing temperatures represent a compromise between increasing production rates at a given reactor size, which is favored at higher reaction temperatures, and reducing catalyst deactivation, which is minimized at lower reaction temperatures. More active catalysts permit operation at lower reaction temperatures, increasing catalyst life and/or decreasing the need to recycle ammonia or trimethylamine.
A number of references disclose methods of making and using zeolites which have been coated with silica, alumina, or like materials. For example, Lindsley, U.S. Pat. No. 3,753,929, describes a method for preparing an alumina-coated zeolite by contacting a zeolite with a soluble aluminum sulfate, or aluminate, at pH 3-5. Nozemack, U.S. Pat. No. 2,079,737, discloses a method for making an alumina-coated zeolite by adding an aluminum salt to a slurry of a finely divided zeolite at pH 7 to 8, and then adding a base to adjust the pH to 9 to 11. The resulting catalysts are claimed to be useful as selective cracking catalysts.
Rollman, U.S. Pat. No. 4,203,869, describes methods for making zeolites having an essentially aluminum-free outer shell, involving depositing an isocrystalline layer of aluminum-free zeolite over the surface of ZSM-5 type zeolite. This catalyst type is also employed in refining processes.
Chu et al., U.S. Pat. No. 4,275,256, disclose a process for conversion of aromatic compounds to dialkylbenzene compounds rich in the 1,4-dialkylbenzene isomer. This process employs a modified zeolite catalyst which has been treated to deposit minor amounts of manganese and/or rhenium, and optionally phosphorus, upon the surface of the zeolite. Chu et al., U.S. Pat. No. 4,278,827, disclose an analogous process which employs a zeolite modified by deposits of minor amounts of germanium, tin and/or lead, and optionally phosphorus, upon the zeolite surface.
Herkes, U.S. Pat. No. 4,283,306, discloses novel crystalline silicas used as catalysts for alkylation of aromatics which incorporate such compounds as arsenic oxide, phosphorous oxide, boron oxide, antimony oxide, amorphous silica, alkaline earth metal oxides, carbonates, and precursors and mixtures thereof.
Rodewald, U.S. Pat. No. 4,402,867, discloses a method for making a zeolite having amorphous silica deposited within the zeolite framework. The resulting catalyst is reported to be useful in such processes as conversion of methanol and dimethylether to a hydrocarbon mixture rich in ethylene and propylene.
Yang, U.S. Pat. No. 4,452,909, discloses a process for preparing silica polymorphs having an outer coating of amorphous silica. Coated zeolites are described; however, their use in methylation of ammonia is not disclosed.
As the foregoing discussion suggests, new catalyst types or process improvements which optimize production of dimethylamine while suppressing production of trimethylamine in the reaction of methanol and ammonia are of interest to the chemical industry.